Rename AutoIncrementing to Counter (#1307)

contracts/mocks/CounterImpl.sol

@@ -0,0 +1,21 @@
+pragma solidity ^0.4.24;
+
+import "../utils/Counter.sol";
+
+
+contract CounterImpl {
+  using Counter for Counter.Index;
+
+  uint256 public theId;
+
+  // use whatever key you want to track your counters
+  mapping(string => Counter.Index) private _counters;
+
+  function doThing(string key)
+    public
+    returns (uint256)
+  {
+    theId = _counters[key].next();
+    return theId;
+  }
+}